Thursday, July 17, 2008

Global marine diversity patterns over Earth's history (or: a cool paper I finally had time to read)

I'm taking a break from commenting on the perceived absurdities in the world around me to focus on a cool paper I read recently - after all science is what keeps me in academia despite the absurdities. The cool paper I'm going to focus on is:

To understand why this paper is so interesting (to me) requires a little background. Back in the 1980s, Dr. Jack Sepkoski (an eminent paleontologist at the University of Chicago) scrounged together as much fossil data on marine invertebrates as he could get his hands on and plotted a graph of global diversity over the past 500 million years or so. (I'm unclear on protocol/legality for inserting graphs from published works into blogs, so I'll provide links to sources as best I can; for Sepkoski's curve click here). One of the striking things from Sepkoski's curve is that global diversity of marine invertebrates appeared to plateau for around 200 million years (from the end of the Cambrian (Cm) to the end of the Permian (P) and then increases dramatically after the big mass extinction at the end of the Permian. If you're interested in what controls global levels of species richness, this graph is a big deal. Afterall, dramatic changes in global richness may provide insights into whether there are limits to the number of species the planet can support and what those limits may (or may not) be.

Sepkoski's dataset was pretty impressive for its day, but most of the biological sciences have undergone a technological revolution since the 1980s. (Paleontology is actually a geological science but I've always thought of them as our long-lost biological cousins). Data is being increasingly concentrated in comprehensive digital datasets and this is equally true for paleontology. The Paleobiology Database (PBDB) has been working to gather "taxonomic and distributional information about the entire fossil record of plants and animals" (quote is from their website). Back in 2001 the PBDB group had a paper (Alroy et al 2001 Proceedings of the National Academy of Science, USA 98:6261-6266) which used advanced statistical techniques to deal with some of the sampling problems with the fossil record (i.e., older rocks - thus older fossils - are rarer then newer rocks and fossils...there may be other biases too, but you should ask go ask your neighborhood practicing paleontologist). They didn't yet have enough data compiled to tackle all of Sepkoski's curve, but they did have data to look at diversity trends from two time periods: 450 million years ago (MYA) to 300 MYA and about 150 MYA to about 25 MYA. (For those of you who have no reference point for what those dates really mean, a good reference point is that the dinosaurs went extinct around 65 MYA). Their results suggested that marine diversity in the more recent time period was no different from diversity 400 MYA!! This was pretty mind blowing if for no other reason than it suggested that marine diversity hit some cap 400 million years ago and has been wobbling around that level ever since.

So, now we've caught up with the paper just published in Science. The PBDB has had an additional 7 years to gather more data to bring to the question - how has global diversity in oceans changed through time? Has the number of different groups of organisms continued to increase since life began or does there seem to be some global limit to the diversity supported globally in marine ecosystems? The answer? Somewhere in between....The new graph shares a lot of similarities with Sepkoski's curve, except that the steep rise of diversity in more recent times is dampened. A lot. In fact, to the point that the interpretation may depend on whether you are a glass half empty or half full kind of person. Diversity in the more recent time spans is higher than earlier diversity peaks (~30% higher). Alroy et al interpret their new results as reflecting a constraint in global diversity, since diversity is not much higher than past levels and does seem to have flattened out in the most recent time periods. On the flip side, in the news of the week article in Science, there are quotes from other paleontologists who think additional work is needed on the recent time periods to really sort things out because there are different preservation issues with "young fossils" that could be underestimating recent diversity. In other words, the sharp rise in diversity in Sepkoski's curve could make a comeback. Not being a paleontologist, I have no comment on that, but I am really looking forward to seeing how the new curve stands the test of time. After all, what is cooler than whether or not there is a global cap on diversity on our planet?